NonProbability Sampling

When conducting a poll, the most widely known technique is the probability method. Using probability acquire respondents simply means leaving the selection of possible respondents to chance. Every person gets the same probability of being selected, and in theory, this will generate a random sample of people, and the results will accurately reflect on the population as a whole. Although probability sampling sounds good, it is often not the best way to conduct an accurate and efficient survey.

Nonprobability sampling leaves the researchers more control over the people they wish to study, and (when conducted properly) is often a more accurate survey than probability.

Nonprobability Sampling gives the researchers control over who they would like to survey, and who they feel would give them the best gauge for public opinions. It also can increase the ease at which researchers can generate respondents. The first type of Nonprobability Sampling is using readily available subjects. This is the most common method of Nonprobability Sampling, and allows researcher to simply go after whatever sources of respondents are available. This can be a quick way to get inaccurate information, however, because researchers may limit the scope of their subjects. For instance, if a researcher wanted to know how students like the computer labs on campus, using readily available students at the computer labs would probably gather a bias for the computer labs. The students who feel the labs are crowded, loud, confusing, etc., would probably not be found using the computers.

The next type of Nonprobability Sampling is known as Purposive, or Judgmental Sampling. This method allows the researcher to select the subjects that the researcher feels would most accurately reflect the opinions of the desired population. For example, if you wanted to know the opinions of conservative minded students, you may interview members of the college Republicans group. This would save the researchers time and effort, but may also lead to a narrow result that doesn’t reflect all conservative minded students.

Snowballing is used when respondents may be hard to come by, or are not readily available. Snowballing lets the subjects help the researchers, by locating other potential respondents. Snowballing can be problematic, however, if your respondents only recommend like-minded individuals, not people who may have different opinions.

Quota Sampling may be the most complex of all the Nonprobability Sampling. Quota Sampling requires research on the desired population, and then applying that research to the selection of respondents. If you were conducting research opinions among students at the University of Iowa, and your research showed that 36% of the students are freshmen, your respondents would likewise need to be around 36% freshmen.

I feel that Nonprobability Sampling has its uses, but should be used only when the need for one or more of the techniques arises. All of the techniques have their places in public opinion research, but researchers should be wary of the pitfalls that each technique holds. In particular, I can see glaring problems with Reliance on Available Subjects and Snowballing. Both are techniques that help the researchers gather respondents, however, both can lead to inaccurate information because of lethargic practices.

In a story posted on Gallup.com, researchers found that support for sending more American troops to Afghanistan had risen from 41% on September 22-23, 2009, to 48% on October 6 2009. This survey comes on the heels of a top ranking American military official publicly stating he recommends an increase in troops. At the end of the story, the writer describes the survey methods, which consisted of 1,007 respondents, interviewed over phone calls. This was Nonprobability sampling, however, because the respondents had to be 18 years of age of older. Perhaps if they had allowed 16 and 17 year-olds to respond they would have found different results. After all, they will soon be voters, and researchers should realize that their opinions can have an effect on the next elections.